There are two types of keyframes:
a)Flash timeline keyframes: e.g. for timeline based motion
tweens.
b) Video keyframes: seekable points inside the flv.

I assume you're talking about (a) above: If you want to have
flash keyframes synchronise with the video... then its probably
more of a flash question than an actionscript one if I understand
what you want to do correctly.
Since flash MX, you can import a video into the library so it
essentially becomes part of your swf and then you can use it like
any other movieclip, but it comes at a cost:

no independent frame rate - the video will play at the frame
rate of your published swf. It can be difficult to maintain audio
synch for longer videos (I haven't done this for ages, but this is
what I can remember about it)

But it does mean that you can use it in Flash as - for
example - the background to some keyframe based motion tweens that
synchronise with the video frames.

Is that what you were wanting to do?
-----------------------------------------
If you were actually wanting to add video keyframes into the
flv itself, that's a completely different exercise and - if they're
not already at 1 second intervals, then you need to re-encode the
video so that they are.

Make another layer in the timeline and put your own keyframes
in where ever you want. If you want them to be every second then
put one keyframe for every x number of frames where x is your
movies frame rate.